Up to 20,000 children were sexually abused by 800 Roman Catholic priests or lay workers in the Netherlands since 1945, an independent inquiry has estimated.

The investigation by Wim Deetman, a former Dutch minister, received 1,800 reports of sexual abuse by clergy or volunteers within Dutch Catholic dioceses, congregations and religious orders. At least 105 abusers are still alive.andy rain

Children involved in Church organisations were twice as likely as non-Catholics to be exposed to abuse and the "mild, severe or very severe sexual behaviour" was covered up by senior clergy.

"The problem of sexual abuse was known in the orders and dioceses of the Dutch Catholic Church," the inquiry concluded. "No adequate action was taken, nor was sufficient attention devoted to victims."

Based on a survey of more than 34,000 people, the 1,100-page Deetman report estimated that one in five children in Catholic school institutions between 1945 and 1985 suffered abuse, twice the average level in the general Dutch population.

Allegations of abuse by the Salesian Fathers at the Don Rua boarding school Heerenberg in the 1960s triggered the wider investigation in March 2010 of paedophile assaults within the Church, a process mirrored in Belgium, Ireland, Germany, Australia, Canada and the US.

The Deetman commission began work last year and as evidence of widespread abuse emerge the Dutch Roman Catholic Church last month set up a compensation system based on the severity of abuse suffered. The total bill for the church could be higher than £4 million.

Over 2,000 people have now registered abuse with the Church and Dutch authorities and a number of cases will be taken to court.

"To prevent scandals, nothing was done: it was not acknowledged, there was no help, compensation or aftercare for the victims," said Mr Deetman.

"There was a policy of 'not hanging out the dirty washing. There is a cultural silence."

Guido Klabbers, from the Dutch KLOKK group of child abuse victims, said: "Everyone can be shocked that this history has come in this magnitude. Everyone can be taken aback that the Church has lied about this and covered it up."

The inquiry did not find a direct link between the Catholic requirement for clerical celibacy and the sexual abuse of children but Mr Deetman did conclude that "sexual need" was a factor.

"We do not consider it impossible that a number of cases would not have happened if celibacy was voluntary," he said.

The inquiry's conclusion that the covered up child sex abuse is not unique to the Catholic Church, another Dutch commission is currently investigating the role of social services in placing children in institutions and foster homes where they were open to abuse.